I had to replace my gas LP water heater a have natural gas in my house,my old tank had a regulator in line on it can that regulator be used on the new tank

Can you re-phrease your question. I can't make any sense out of it.

Generally changing the gas type requires purchasing a conversion kit which contains new burner and pilot orfices. Sometimes certain devices with built in regulators either need to be changed or re-configured.